The fluidized catalytic cracking of hydrocarbons is the main stay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils. Large hydrocarbon molecules, associated with the heavy hydrocarbon feed, are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield of gasoline products.
The basic equipment or apparatus for the fluidized catalytic cracking (hereinafter FCC) of hydrocarbons has been in existence since the early 1940's. The basic components of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by counter-current contact with stream or another stripping medium. The FCC process is carried out by contacting the starting material whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons with a catalyst made up of a finely divided, particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. During the cracking reaction, coke will be deposited on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content, relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature of the catalyst, activity of the catalyst, quantity of the catalyst (i.e., catalyst to oil ratio) and contact time between the catalyst and feedstock. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone which simultaneously produces a variation in the catalyst to oil ratio. That is, if it is desired to increase the conversion rate, an increase in the rate of flow of circulating fluid catalyst from the regenerator to the reactor is effected. Since the catalyst temperature in the regeneration zone equilibrates at a significantly higher temperature than the reaction zone temperature, increasing the catalyst flux from the relatively hot regeneration zone to the reaction zone while maintaining a constant feed preheat temperature effects an increase in the reaction zone temperature.
As the development of FCC units has advanced, temperatures within the reaction zone were gradually raised. It is now commonplace to employ temperatures of about 525.degree. C. (975.degree. F.). At higher temperatures, there is generally a loss of gasoline components as these materials crack to lighter components by both catalytic and strictly thermal mechanisms. At 525.degree. C., it is typical to have 1% of the potential gasoline components thermally cracked into lighter hydrocarbon gases. As temperatures increase, to say 1025.degree. F. (550.degree. C.), most feedstocks can lose up to 6% or more of the gasoline components to thermal cracking.
One improvement to FCC units, that has reduced the product loss by thermal cracking, is the use of riser cracking. In riser cracking, regenerated catalyst and starting materials enter a pipe reactor and are transported upward by the expansion of the gases that occurs upon contact with the hot catalyst and is the result of vaporization of the feedstock hydrocarbons, and any fluidizing medium that may be present and reaction of the feedstock hydrocarbons. Riser cracking provides good initial catalyst and oil contact and also allows the time of contact between the catalyst and oil to be more closely controlled by eliminating turbulence and backmixing that can vary the catalyst residence time. An average riser cracking zone today will have a catalyst to oil contact time of 1 to 5 seconds.
Better control of contact time and a reduction in backmixing is obtained by promoting good initial mixing and rapid vaporization of the feedstock. Atomizing feed distributors are now commonly used to improve the dispersion of feedstock into the fluidized catalyst stream. Raising the temperature of incoming feed prior to contact with the catalyst also improves the rate and extent of vaporization. Hence, a hotter feed can promote better feed distribution and contact time control.
A hotter feed can also facilitate the use of higher reaction zone temperatures. By heating the feed, reaction zone temperatures can be increased without circulating additional hot catalyst from the regeneration zone. However, externally heating the feedstock will put additional heat into the FCC system. A portion of the added heat gets transferred to the regenerator and, unless removed, will raise the temperature of the regenerated catalyst. Therefore, absent some form of additional heat removal, heating the feed will alter the heat balance between the reactor and regenerator thereby requiring a change in the catalyst to oil ratio. Accordingly, in some cases, heating the feed can decrease the catalyst to oil ratio to unacceptably low levels. As a result, it is highly desirable to have a means for removing heat from the regenerator so that the catalyst to oil ratio and heat balance can remain substantially fixed while injecting feed at a higher temperature and maintaining a constant reactor temperature.
Catalyst coolers are a well known method for removing heat from FCC regenerators. These coolers remove a portion of the heat that evolves in the regenerator from the combustion of coke so that catalyst and regenerator temperatures remain within acceptable limits. As FCC feedstocks become heavier, cooler use has become more widespread. One popular type of catalyst cooler, generally referred to as a remote cooler, has a series of indirect heat transfer tubes contained in an exchanger vessel situated outside the regenerator. Catalyst circulates between the exchanger vessel and the regenerator, while a cooling medium, usually water and saturated steam, passes through the cooling tubes. Most catalyst coolers employ dense phase conditions and are either of the flow-through or backmix type. FCC feed temperature is externally controlled before it is added to an FCC reactor; maintaining a constant regenerator temperature with increasing feed temperature requires additional catalyst cooler duty to remove the added heat input from the feed. Providing a heater to raise feed temperature and a catalyst cooler to then take the heat out is thermodynamically inefficient and adds to the cost of operating the process.
It has been suggested in U.S. Pat. No. 2,735,802, issued to Jahnig, that oil feed may be preheated in a flow-through-type catalyst cooler. Using a catalyst cooler to heat feed, advantageously shifts the heat balance in the FCC system to raise the feed temperature without generating additional heat for removal in the regenerator. It is generally known, and acknowledged in Jahnig, that preheating of an FCC feedstock can pose cracking and coking problems. Raising the temperature of the feed above 700.degree.-800.degree. F. promotes thermal cracking of the feed which, unlike catalytic cracking, causes the random cracking of hydrocarbon chains and the polymerization and dehydrogenation of higher molecular weight hydrocarbons. These reactions reduce liquid product yield and increase the yield of light gases and coke. Hence, surface temperatures of the tube walls must be carefully controlled in order to avoid temperatures that can result in thermal cracking. A flow-through type cooler, as shown in Jahnig, passes catalyst into the heat exchanger tubes at one point and withdraws catalyst from the heat exchange tubes at a different point. With this type of catalyst flow, the tubes at the point of catalyst entry are exposed to higher temperatures than the tubes at the point of catalyst withdrawal. Such temperatures gradients in the catalyst that contact the tubes can result in locally hotter tube temperatures, particularly in view of the relatively low heat transfer coefficient between the tubes and the oil. Therefore, while the average temperature of the oil as it leaves the cooler may not promote thermal cracking, the flow-through design can thermally crack the feed where it contacts localized hot tube sections.
It is an object of this invention to offer an improved method of heating FCC feed in a remote catalyst cooler.
Another object of this invention is to provide a remote catalyst cooler of simplified design for preheating an FCC feed.
It is another object of this invention to provide a remote catalyst cooler that specifically suits the duty requirements for heating an FCC feedstock.
It is a yet further object of this invention to provide a process for heating FCC feedstock in a remote catalyst cooler that offers improved control of the temperature of the surface of heat exchange elements.